It is commonly known that semiconductor substrates may be manufactured by means of processes like depositing thin films on surfaces of silicon wafers. Such processes may be carried out in a vertical furnace. Such a vertical furnace is known and typically comprises a process chamber arranged in said furnace such that a central axis of the process chamber substantially coincides, or at least extends in a substantially similar direction, with a central axis of the vertical furnace. Vertical furnaces usually have a load size of 100-150 wafers that are spaced apart such that during processing of the respective wafers the entire surface of each substrate can be subjected to the process. Before processing wafers in a vertical furnace, the wafers are provided in a loading configuration wherein the wafers are substantially horizontally oriented and are mutually spaced in a vertical direction wherein the centre points of the wafers are approximately positioned on the central axis of the process chamber. The loading configuration may be provided by a wafer boat that is adapted to receive a plurality of wafers. In a number of film deposition processes, the film deposited on the wafers may be relatively thick at an edge region of the wafer. To improve the uniformity in thickness of the film deposited on the wafers, the wafer boat may be covered by a boat cover that substantially surrounds the entire wafer perimeter. The cover comprises a pattern of openings, for instance circular holes or elongated slits, to allow gas to enter the boat such that all wafers to be processed may be fed with a uniform reactive gas stream.
From U.S. Pat. No. 5,556,275 a heat treatment apparatus comprising a boat for holding a plurality of disk-shaped objects such as wafers is known. The wafers are arranged coaxially and spaced at predetermined intervals. The heat treatment apparatus further comprises a hollow cylindrical cover having a plurality of gas ports. The cover may comprise a plurality of parts. For instance, two of the parts may be secured to the boat. To load the wafers into the boat, the wafers have to be inserted through one of the two openings provided between the respective cover parts. The openings between the cover parts mounted to the boat are covered by means of two removable cover members. The first removable cover member preferably has an arcuate cross section and the second removable cover member may comprise a flat plate. To load the boat with wafers, the boat that is provided on a pedestal may be removed from the processing chamber by means of a boat elevator. Wafers may be loaded into the boat by a wafer loading device adjacent the boat elevator. When all wafers have been set into the boat, the removable cover members may be attached to the boat covering the openings between the secured cover parts. After attaching the removable cover members to the boat, the boat is elevated such that is brought into the processing chamber.
From U.S. Pat. No. 5,902,103 a vertical furnace is known for use in a semiconductor manufacturing apparatus which comprises a heater, an outer tube, an inner tube and a boat adapted to be introduced into the inner tube with a wafer loaded thereon. The boat comprises a boat cover disposed permanently internally of the inner tube concentrically therewith. The boat cover comprises a boat cover body having an amount of slit apertures extending in a longitudinal direction thereof. The boat cover further comprises auxiliary cover plates connected to said boat cover body such that the slit apertures are covered thereby providing a gas flow passage between the auxiliary cover plate and the boat cover body. The boat cover may be separable in two semi-cylindrical halves, one of the halves residing in the inner tube permanently. The boat is composed of a ceiling plate, a bottom plate and support posts connecting said plates. The support posts are provided with grooves with a predetermined pitch to retain semiconductor substrates.
The boat cover may thus, at least partially, be mounted within a tube, such as an inner tube of the vertical furnace. To load/unload wafers into/from the boat, the boat provided on a pedestal may be removed from the inner tube of the vertical furnace, via a loading opening in the bottom of said furnace whereas the boat cover remaining inside the inner tube. The boat on the pedestal may be filled with the silicon wafers to be processed and the loaded wafer boat may be inserted in the inner tube of the furnace again, such that the boat is covered by the boat cover in the inner tube. The substrates are then ready for processing.
A disadvantage of this design is that the clearance between wafer edge and cover, required to allow insertion of the wafer boat into the inner tube without scratching the inner tube, would be significantly larger than the clearance that is needed to obtain optimum uniformity of film thickness on the wafer. In an alternative design, covers are mounted manually on the boat. Although this design allows a smaller clearance between wafer edge and cover, the manual handling of cover plates is not compatible with a manufacturing environment. Automation of the cover placement would require complex robot systems that significantly add to the complexity of the entire system.
Due to an increasing demand of thin semiconductor substrates, it is desired to enhance the throughput of substrates to be processed in a vertical furnace and to have a production worthy solution for processing wafers in a cover boat.
Therefore, it is an object of the present invention to provide an improved loading apparatus for loading planar thin semiconductor substrates in a cover boat in a vertical furnace. More in particular an object of the invention is to provide a simple loading apparatus for loading semiconductor substrates in a cover boat in a vertical furnace in a fully automated way wherein a loading time is minimized and wherein a small clearance between wafer edge and cover can be obtained without a risk of scratching of the process chamber wall by the wafer boat during insertion of the wafer boat into the furnace.